Realization of extremely narrow divergence angle and ground test method toward quantum key distribution based on a medium-high orbit satellite.

The medium-high orbit quantum science experimental satellite is designed to conduct quantum communication experiments over distances of 10,000 km both during daylight and at night to establish a global quantum communication network and enable multiple quantum experiments. To ensure efficient links over such vast distances, a beam divergence of 3 µrad at 850 nm is required-posing significant challenges in designing lightweight, large-aperture telescope systems and detecting ultra-narrow divergence angles. Here, through analysis using a far-field diffraction model, we determined that the optimal aperture should be 660 mm and that the RMS wavefront aberration must be controlled to 1/9λ at 632.8 nm. We employ multi-stage beam expansion to achieve waist diameter matching, select silicon carbide as the primary mirror material, and optimize the system's structure and installation methods to reduce weight while maintaining surface accuracy. Ultimately, the system achieves a surface accuracy of 1/10λ at 632.8 nm, with the divergence angle measured at 2.9 µrad at 850 nm and 4.75 µrad at 1550 nm-approaching the diffraction limit for this aperture size. Additionally, a 10 m coaxial Cassegrain system, combined with gravity unloading and beacon spot tracking techniques, mitigates the impacts of gravity, airflow, and temperature on detection. These achievements lay a solid foundation for efficient quantum communication.